Hempcrete PassivHaus (build in Australia)

[Aus_Doug]

Hi,

just joined because I was reading all the posts about MHRV. I have the basic design of my proposed 110 sq M house: 3br, 2 disability bathrooms, superinsulated, Off centre roof pitch with the long side facing N (S hemisphere!).

 wrt the MVHR, it seems the most economical solution for me is a Mitsubishi Lossnay ceiling mounted. After doing some research, I have found a Chinese Co that supplies 90mm round Semiflex duct & accessories. Yet to create a manifest, but providing the freight is reasonable, should be economical.

The Co is: https://www.erviguicoo.com/fresh-air-abs-distributor-with-high-quality-abs-materials-product/. So far I have a catalogue, but will need 

drawings, etc. The Lossnay will be locally sourced for warranty. btw, the Steib-Eltron tool is great for designing the MVHR system.

 

 Some details of the build: light construction because there are no snow loads! The Wall frames will be Pine, with the top & bottom plates screwed as well as nailed (due to Cyclone uplift). Studs 90x45, on 600 centres. The roof trusses will be at 900 centres. The roofing is Trimdek corrugated iron, but I will use 0.48mm as I will use S5 mounts for the solar PV that are rail-less (just screw to the ridges, hence the thicker steel).

Hempcrete: 270mm thick walls with 10mm lime based render each side. One thing I will do differently is to raise the truss bearing point so I can have 300mm of insulation across the ceiling with no step down, The Hempcrete walls will go to the top of the outside edge of the trusses, & will return into the ceiling, sitting on a ledger. Then I can seal & attach my OSB airtight layer directly under the trusses. The OSB also gives wind bracing. (all joints sealed & taped).

 300mm Fibreglass insulation above OSB.

 This will be built on a slab foundation, using Cupolex domes. The edge rebate will have stainless ant barrier/dampcourse.

 Air sealing:  all transitions taped after walls are rendered. Slab is to be polished, so the concrete will be edge primed before taping .

 

Blower Door test: I will DIY a blower door using a s/h Automotive 12v radiator fan. I already own a digital manometer, & after reading the MVHR threads will buy a Testo 425 Digital Hot Wire Anemometer (the computer connected one), & control the fan with a DC speed control to vary output. The Air volume can be easily measured with a cone over the fan, & something like a piece of 150mm plastic stormwater pipe, with a hole to insert the Anemometer probe.

 This anemometer is apparently sensitive enough to check for leaks around windows, etc. The Automotive fan will easily blow/suck 50 pascals.

 

 My build is also in a bushfire zone. No gaps bigger than 2mm in the outer sheath & roofing: I do not use Soffits or barges: I use a product called G&G gutter brackets that screw to the metal roofing overlap. I use 150mm half round gutter that I can easily clean (with a moon section on a pole). The gutter ends are sloped so I can sweep the leaves out. In Australia, the roof space must be ventilated, which is accomplished with about 75mm wide folded perforated aluminium sheet that runs on top of the hempcrete, to the underside of the roofing. I have designed a system that stops the embers. The vent at the ridge is a product called Vent-a-Ridge. This allows air to pass under the (raised) ridgecap, exiting through the corrugations. There will be the usual smart membrane & counter battens for air flow.

 

 Hopefully I can build this for less than a ´normal´ builders build (aiming for $au2500 per M sq)

 

regards, Doug

[Aus_Doug]

One thing I am getting my head around is the need to DESIGN EVERYTHING! Even the hole thru the wall (requiring sealing) for the porch light. Most electrical work will be inside the sealed envelope (which is the interior lime plaster in a Hempcrete house). Electrics will pass from the ceiling cavity (~125mm also used for MVHR) through a conduit to the cable end (switch or power point for example), so that conduit passes through the rendered membrane, down the wall, then back thru the membrane, all requiring sealing. Australia does not have any decent sealed 

wall boxes, so I must work out how to effectively seal the available boxes. Of course all of this will be installed before the Hempcrete placement.

 Even the MVHR air supply return will need to be installed (or at least arranged for) before the Hempcrete placement.

 I am thinking of ´trussing´ up the window & door heads instead of using the usual timber lintels. Accomplished by diagonal struts between studs over the window/door. I will be having the frames factory made, so I can either ask them to fit the bracing, or retrofit on site. One thing with Hempcrete is that the noggins (horizontal timber between studs) are fitted not on the flat, but rotated 90 degrees (ie 90mmx45 studs, noggins similar size but 90mm is vertical). This makes it easier to place the Hempcrete. Remember we have no snow loads, so the frames are much lighter in Australia. Usually in Australia the wall frames are braced with ply or OSB. With Hempcrete, the ply is not used, so needs to be replaced by timber bracing. In my case, with 90mm studs, & 270mm Hempcrete I have 90mm each side of the frame. The Bushfire certification needs at least 75mm of Hempcrete, so the bracing is internal. The bottom wall plate is 45mm, sitting in a ~40mm slab rebate, so my solution is to double up the bottom plate so I can have room to attach the required bracing (probably 75x25mm pine bracing, but up to the Engineer!). Being in a sub tropical zone, we need hold down for the wind load, so the top & bottom plates are screwed to the studs as well as being nailed. The bottom plates are Concrete screwed to the slab, & the roof trusses are steel strapped to the wall frames. As I said previously, there will be an OSB layer under the trusses sealed as the air seal membrane, but als giving wind bracing. The internal walls are non-load bearing (basically ending at the celing cavity so services pass over)

 An explanation of the No soffits, no Barge board: basically roofing iron can self support for 300-400mm, including the guttering load (no snow!). The 600 eaves are supported by a purlin supported on steel  brackets off the wall. No timber due to the Bushfire zone! All the mounts for these brackets also need to be designed & fitted pre Hempcrete. I am proposing using bolts that are a woodscrew one end, & Metric thread the other. Extended by a loctited double nut & extra thread. These can be found with a stud detector after the wall is hempcreted & lime rendered.

[Iceverge]

Welcome welcome. 

 

Nice to have someone from down under and to learn from your particular situation. 

 

A friend of mine built a hemp Crete house recently but from blocks in Ireland. 

 

Lime render

75mm hempcrete blocks

170mm hempcrete batts in a stick built timber frame. 

75mm hempcrete blocks

Lime render. 

 

I don't know if the blocks are available on Oz but it was less labour intensive than shuttering on site hempcrete. 

 

For the TF have you considered stick built on site?  I like the hammer band they use in Sweden rather than a double  top plate. Maybe it would solve your lintel issue. See 9mins20secs into the vid. 

 

 

https://youtu.be/4nRz0D0bNmg?si=qOs-IhmdRxzuLNad

 

[Aus_Doug]

Thanks for that. I was aware of similar techniques. (We would probably call that a Ring beam, but usually 4 sides).

 

 The difference with ´trussing ´ the frame above the windows/doors is it uses really cheap timber: probably 50x35, because it is in compression. 

Anything bigger than 90mm in Aust. gets very expensive (comparitively). I can also potentiall recycle some pine Pallet timbers. I will give a drawing of my roof trusses later, but basically they step down 90mm from the wall plate. This gives me plenty of room for the Hempcrete up to the roof membrane (I will use Fibre cement at the bottom of the pitch to have something to form the Hempcrete against). The idea is to have the full 270mm on radius for my Hempcrete (so no cold spots). By having my Ceiling insulation transition inset into the room, I can better manage the gaps etc. btw, the double top plate is needed because the trusses are not always in alignment to the studs. Also using small timber members gives better adhesion of the hempcrete. (Large beams need holding studs such as a large plastic washer on a spacer so the hempcrete does not separate.) I have tried to use the minimal amount of (purchased) timber that I can to reduce costs.

 

 Shuttering Hempcrete is fairly easy: 10mm OSB is adequate. You screw a long screw into the joist to the required thickness of hempcrete, then screw the shuttering next to the spacer screw. The top can be held with timber spacers that are removed when the fill gets near the top. After the Shuttering removed, remove the spacer screws & fill the holes. Pretty easy. Sometimes one will need to work around fixtures etc, but all do-able I feel.

[Aus_Doug]

Just for an idea on timber price: 90x45mm studs are $au 5.85/M, where 190x45mm is $au17.05/M. That is the reason I think trussing will be worthwhile....

[Iceverge]

That's over double the price in Ireland. 

 

 

[Oz07]

900 centres on trusses interesting. If it has design implications is it not worth reverting to 600 cc or does cost work out a lot better on 900?

[Aus_Doug]

Because it is a minimalist build, 900 trusses is worthwhile. As I said the only roof loads are solar panels * perhaps a HWS in the ceiling.

 Timber is expensive in Australia because most is imported. I will try to minimalise by buying sling lots/packs but not always possible.

 

 I have clipped a detail drawing of the top of the wall cross-section: I had a think on the idea of a big member at the top of the wall, then realised I could run it outside the end of the trusses. The 190x45 would be glued & screwed to the 90x45 top plate. The trusses are then screwed thru the end, which also gives some bracing. I might even get away with not tieing the trusses down with steel strap (reducing thermal bridges). I will need

to discuss this with the engineer... The truss is 225mm high at the end.

 One other note: I returned the Hempcrete into the room that will help the insulation transition. I will nogg between the trusses at the edge of the OSB to support the edge. The OSB will be taped to the Wall render for the air tight layer. I will seal the OSB with Waterglass to reduce porosity. 

 Also note the layer of (450mm w) Fibre Cement sheet under the counter batten at the lower edge of the truss: that gives a permanent form for the Hempcrete. It also gives a flat sheet to Fire seal the vent that is not detailed there.

Cross_sect_Wall_top.pdf